Charging a Discharged LiPo Battery?

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Hi

I have 2 LiPo batteries in series, and have used a commercial charger to charge them up (SMART 2020). This works well.
What I did then was disconnect the wall cube but left the batteries plugtged into the charger.
Two days later I checked the voltage on the 2 batteries in series and it was 1.5V!!!

The charger must have DRAINED my batteries.

The problem now is that if I plug in the wall transformer (power the charger), it reports an error upon charging. It is incapable of charging the batteries!

My questions are:
(a) What can I do when the LiPo's are over-drained?
(b) Is it common for a charger that is not powered to drain the batteries?

Thank you in advance.
a.

 

[Hero999]

It looks like they're beyond the point of no return.

You could try trickle charging them a bit before connecting them to your real charger.

 

[jpanhalt]

The usual recommendation is not to discharge below 3.1 or 3.2 V per cell. I suspect you know that.

Charging over-discharged cells can be dangerous. If you do decide to charge them and get any puffing at all, I would recommend throwing them away. I would also strongly recommend that you do the charging in a place where a small fire will be contained. For myself, I would not try to charge them. There are just too many reliable stories of adverse (fire) things that can happen.

I just threw away a 3S pack ($70) after one use, because the cells puffed. So far as I know, the cells were never discharged below 3.8V and the charger is one of the better chargers. That happens. John

 

[audioguru]

I think a lithium rechargeable battery that is discharged below 3.2V is unstable and can catch on fire at any time. Lithium burns with a white-hot flame like magnesium (used in fireworks). Water makes it hotter. Don't risk a fire.

Dispose the battery in a hazzardous products disposing place.
The charger that discharged the battery is defective and cheap. Fix it or throw it.

 

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Hi All

Thank you for your advice.
We will most probably end up chuking the batteries out.
But before we do- can you please tell me how to trickle charge the batteries? Can I use a Power Supply where I can set the current?
What is a better way of doing it?

The interesting thing is that I baught the SMART 2020 charger from
www.anyrc.com.
It was not cheap- that is for sure!
Can someone recommend a simple 2 cell charger (2 wires only), to charge 170mAhr batteries?

I appreciate your help.
a.

 

[audioguru]

Look at battery charger ICs from Maxim. They charge a Lithium battery at a very low current until its voltage reaches the 3.2V minimum for a Lithium battery to be charged normally.

 

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Hi AudioGuru

Thank you for your reply.
I have used the MAX1555 for a single cell battery very succesfully.

However now I have 2 LiPo batteries in series, and could not figure out how to use the MAX1555.
Can you recommend something as simple as the MAX1555 for two batteries in series?

In the mean time can I use a power supply in which the current is set?

Thank you
a.

 

[audioguru]

Batteries are not exactly the same. If you charge them in series then one will become overcharged and the other will be under-charged. It gets worse when they discharge. Then the weakest cell gets dead and the remaining discharging cells charge it in reverse which kills it.

 

[jpanhalt]

But before we do- can you please tell me how to trickle charge the batteries?

It is a bit of an oxymoron to ask for the correct way to do something that can't or shouldn't be done.

Here are two links from the British and American model aircraft groups that strongly advise against doing what you seem to want to try:

https://www.rchobbies.org/lithium_battery_breakthrough.htm
**broken link removed**

The charging profile for LiPo batteries is described here:
" These chargers give a constant current charge rate at 75% the cell capacity until the pack voltage reaches 3.6 volts per cell. This charges the pack to about 80% of total capacity. At this point the charger switches to a constant voltage charge rate of 3.6 volts per cell to top off the battery. To charge a fully depleted pack typically takes about one and a half hours. " (Source: RC Hobbies)

All that being said, however, I think you should toss the batteries and get new ones. John

 

[audioguru]

The charging profile for LiPo batteries is described here:
" These chargers give a constant current charge rate at 75% the cell capacity until the pack voltage reaches 3.6 volts per cell. This charges the pack to about 80% of total capacity. At this point the charger switches to a constant voltage charge rate of 3.6 volts per cell to top off the battery. To charge a fully depleted pack typically takes about one and a half hours. " (Source: RC Hobbies)

They are WRONG!
A Li-po battery cell is charged with a 4.2V source. The charger's current is limited to the capacity rating of the battery cell. When the cell's voltage reaches 4.2V it is about 75% charged then it will draw less current. When the current drops to about 0.01 of the cell's capacity then it is fully charged and must have the charger turned off.

 

[jpanhalt]

I don't know either way which is correct. The Kokam battery producer states on its site: https://www.kokam.com/english/biz/rc.html

"Similar to other lithium rechargeable batteries, SLB452128 has typical charge discharge characteristics. SLB452128 can be charged by simple charging process with constant current at the beginning of the charge cycle and followed by constant voltage at the end of the charge cycle."

Are there other reliable sources that suggest a constant voltage charge for the entire charging cycle? John

 

[audioguru]

The Kokam battery charger is also 4.2V that is limited to the max charging current you want.

The charger reduces the voltage when it limits the current. As the battery charges it uses less current so its voltage rises from the constant current charger. If the voltage becomes higher than 4.2V then the battery over-charges and catches on fire.

 

[jpanhalt]

So, is the difference between what Kokam and RC Hobbies say and your comment the distinction between "constant current" and "current limited?" What is the practical difference between the two? I suspect "constant current" chargers are also voltage limited.

I use a "sophisticated," LCD-screened, expensive charger, and it still puffs the batteries. And that was starting with a pack at 3.8V per cell. Can't wait until the weather warms up enough, and I can launch my sailplanes in the usual way. LiPo's are great power to weight, but a real expensive PITA.
John

Edit: corrected LCD screen

 

[audioguru]

Current limited, constant current and regulated current have the same meaning for a battery charger. Then the battery determines the current-limited voltage. The voltage has a max of 4.2V.

I don't have a Li-po battery so I don't know if it is normal for them to "puff".
Maybe your charger has its current set too high.

A Li-po battery charger has its voltage set very accurately to exactly 4.2V. Some can charge Li-po cells in series then the voltage is in multiples of 4.2V.

My RC "glider" has an electric motor driving a folding prop. It is powered from a heavy Ni-MH battery. The plane glides like a brick. It is too heavy to climb high and frequently stalls when trying.

 

[jpanhalt]

My charger is one of those that charges the individual cells through a separate connector. It then balances them, presumably to exactly 4.2V, but I am not sure whether anything else is involved in the balance stage. Among hobbyists, series charging of LiPo packs is quickly losing favor because of the problem of over-charging some of the cells.

With respect to the personal anecdote, the charge current was set at 1.0 A, which was well below the maximum allowed rate of 2.1 A for the cells I was using.

My glider, which has had only one flight after conversion from a pure glider to electric was a dream to fly. It soared as well or better than it did before conversion.

You need another sailplane, rather than fight trying to keep some brick up. For the type of gliders I fly, it is not so much weight as it is drag. They can go quite fast, but lose little altitude. John

 

[sam2]

The cells are ruined.
Dont try to recharge them,If you do, you will have a time bomb on your hands.
They may take a charge and even function normaly for a short time, but they will eventually ignite, and when they do I just hope that you are out flying and not sleeping with the airplane somewhere in your house!

Its not a joke , more garages, houses and cars have burned up from mistreaded lithiums than I can count.

I have used them for years now, and I always store them in a sealed metal amo box in the garage.
I never keep them in my planes when I am not flying.

 

[Oznog]

(b) Is it common for a charger that is not powered to drain the batteries?

Yeah regrettably it's a fairly common problem I hear. Normally it's only a few mA, but your battery's only 170mAH? A few mA would indeed drain that in a couple of days. Of course there are also chargers designed not to do this sort of thing at all too.

 

[adnan_m_s]

Inquiry

Hellwo everyone,
since you are addressing the charging issue could you please give me some information on how to estimate the minimum and maximum current for charging and also the right volatage I am planning to charge a 12 V 12AH battery like scooter batteries,
any good links to estimate these ?